Insulating oil used in an oil-filled electrical device such as a transformer or a reactor may contain traces of elemental sulfur and sulfur compound that are corrosive. Corrosive sulfur reacts with metal such as copper wire coil that is susceptible to sulfuration used in the oil-filled electrical device and generates conductive sulfide. There are many reports that such conductive sulfide causes malfunction of oil-filled electrical devices. The mechanism how the sulfide is generated is not fully clarified, and what is found to date is simply that the mechanism depends on the temperature and the type of insulating oil.
As a method of diagnosing sulfide in an oil-filled electrical device, Japanese Patent Laying-Open No. 4-176108 (Patent Literature 1) discloses the following configuration. Specifically, in the oil-filled electrical machinery, a detecting part having metal particles dispersed on a surface is provided in contact with insulating oil of the oil-filled electrical machinery, and change in electric resistance of the detecting part is found. The metal particles dispersed on the surface of detecting part are combined with elemental sulfur and sulfur compound in the insulating oil, so that conductive sulfide is generated at the surface of the detecting part. This leads to a change in electric resistance of the detecting part and, therefore, by finding the change in electric resistance, it is possible to know the degree of sulfide generation.
Japanese Patent Laying-Open No. 59-6721 (Patent Literature 2) discloses the following configuration. Specifically, in an oil-filled electrical device including an oil-filled electrical device tank containing a main body of the electrical device and insulating oil and a cooling device coupled via an oil pipe so that the insulating oil is circulated, an oil reservoir is provided in insulating oil circulating system, that is, in the body tank of the oil-filled electrical device, in the cooler or in the middle of the oil pipe. In the oil reservoir, a sensitive medium that reacts with active sulfur in the insulating oil is provided, and substance deposited on a surface of the sensitive medium is detected by a sensor including a light-emitting photoelectric device and a photoreceptor. Further, a display device is provided, which receives a signal output from the sensor as its input.
Further, Japanese Patent Laying-Open No. 2-103910 (Patent Literature 3) discloses the following configuration. Specifically, in an electrical device having a main body fixed in a tank filled with insulating oil, a sample storage box is provided, through which the insulating oil in the tank circulates. A conductor sample having the same metal as the conductor of device main body as a main component is placed in the sample storage box. Thus, the conductor sample in the sample storage box comes to be subjected to the same conditions of sulfide corrosion as the conductor of device main body in the tank. Therefore, by monitoring the conductor sample, it is possible to monitor the development of sulfide corrosion of the device main body.
Japanese Patent Laying-Open No. 4-252945 (Patent Literature 4) discloses the following configuration. Specifically, a moisture-in-oil monitor of an oil-filled electrical device is for monitoring moisture in insulating oil of an electrical device having a device main body placed in an oil tank filled with the insulating oil, and it includes: a moisture-in-oil sensor formed of an insulating paper placed between dielectric characteristic measuring electrodes, whose dielectric characteristic changes depending on moisture equilibrium with the insulating oil; a sensor container provided outside of the oil tank, housing the moisture-in-oil sensor, and filled with the insulating oil; a first communication pipe inserted between the sensor container and the oil tank, through which the insulating oil in the sensor container and in the oil tank flow; a first communication valve provided at an end or in the middle of the communication pipe for opening/closing the communication of insulating oil; and a dielectric characteristic monitor part connected to the measuring electrodes of the moisture-in-oil sensor, converting a dielectric characteristic value of the insulating paper to an amount of moisture in oil and, if the amount of moisture in the insulating oil exceeds a reference value, determining occurrence of a malfunction and outputting an alarm signal.